The invention relates to a method and apparatus for regenerating alkanolamine from a mixture containing oxazolidone.
To remove acidic gases such as carbon dioxide (CO.sub.2), hydrogen sulfide (H.sub.2 S), carbonyl sulfide (COS), and the like, from gaseous mixtures there are employed absorption solutions containing alkanolamines. In the absorption process the alkanolamine of the absorption solution reacts with the carbon dioxide of the gaseous mixture to form carbonate, carbamate, and bicarbonate. In the regenerating of the loaded absorption solution there are applied elevated temperatures and pressures which are kept as low as possible. This results in the removal of the acidic gases from the absorption solution and reformation or regeneration of the free alkanolamine. However, under the conditions prevailing during the regenerating process some of the carbamate reacts irreversibly with the result that an oxazolidone derivative is formed. Thus, free alkanolamine is lost and the absorbing capacity of the solution is reduced. The reaction product oxazolidone concentrates in the circulating absorption solution and separation thereof becomes necessary. The oxazolidone derivative is a waste product. Therefore, fresh alkanolamine in an amount corresponding to the oxazolidone has to be fed to the plant.
There is a method of absorbing acidic components from gases described in U.S. Pat. No. 3,658,462 wherein, with a view to recovering the alkanolamine, those reaction products of the absorption process which are not regenerative are contacted with caustic and a sufficient amount of water is withdrawn from the reaction mixture to decompose the reaction mixture into an alkanolamine portion and a salt portion in aqueous solution. The decomposition, by addition of caustic, oxazolidone into alkanolamine and sodium carbonate is dependent on the addition of considerable quantities of caustic. Moreover, after the recovery of alkanolamine there remain considerable amounts of waste products such as a sodium carbonate containing solution which still contains certain amounts of the components of the gas washing liquid. Furthermore, the caustic must be brought to reaction with the oxazolidone in nearly stoichiometric proportion. If the amount of caustic added is too low, a lower yield will result; if it is too high, this means a loss of chemicals. During phase separation it is necessary to keep the concentration of caustic within narrow limits. If the concentration is too low, much of the alkanolamine will be lost with the sodium carbonate phase; if it is too high, disturbances will occur due to the deposition of solid sodium carbonate. It is true that the concentration of the caustic can be controlled by means of controlling or dosing devices, but such devices are relatively expensive.